Multirange flowmeter

ABSTRACT

A multi-range gas flowmeter having a primary gas flow passage and a secondary passage which houses the flow measuring device and which is connected to the primary passage at its inlet and outlet ends. A multi-position valve is located in the primary passage between the inlet and outlet ends of the secondary passage to divert all or a portion of the flow to the secondary passage, depending upon the value of the flow rate to be measured.

BACKGROUND OF THE INVENTION

This invention relates to a multi-range gas flowmeter. Conventionalmulti-range flowmeters measure the gas flow by sensing parameters of theflow in the main flow path. U.S. Pat. No. 3,321,970 to Walker, and U.S.Pat. No. 3,037,384 to Good, which are directed to flowmeters whichmeasure the flow rate by sensing a pressure differential across anorifice, disclose devices which place different sized orifices in themain flow path to accommodate different ranges of flow rate.

Other types of flowmeters, such as those which sense the change intemperature of the flowing gas caused by the addition of a fixed amountof heat, utilize a bypass or secondary flow path for the measurement ofthe flow rate. Because such a flowmeter, as described in U.S. Pat. No.3,851,526 to Drexel, diverts only a relatively minor portion of the flowthrough the secondary path, the accuracy of the flowmeter degrades atvery low flow rates.

SUMMARY OF THE INVENTION

The present invention is directed to a multi-range gas flowmeter whichincludes two passages for the flow of the gas, a primary passage and asecondary passage which is connected at its inlet and outlet ends to theprimary passage. The portion of the flowmeter which acts as the flowsensing device, which can be of generally any type including thosesensing heat flux or pressure differential across an orifice, is locatedin the secondary passage of the flowmeter. A valve is located within theprimary passage, between the inlet and outlet ends of the secondarypassage, and is used to divert all or a portion of the gas flow from theprimary passage through the secondary passage. With this type offlowmeter, all or a portion of the flow to be measured can be divertedthrough the passage containing the flow measuring device, therebyimproving the accuracy of the flowmeter over various ranges of flowrate.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken into conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a sectional view of the flowmeter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the flowmeter 10 contains a primary flow passage14, a secondary flow passage 16 which communicates with the primarypassage 14 at its inlet end 20 and outlet end 22, a valve 24 fordiverting the flow from the primary passage to the secondary passage,and means for measuring the flow of the gas through the secondarypassage.

In the embodiment shown in FIG. 1, the flow measuring device located inthe secondary passage 16 comprises a pair of conventional resistancetemperature detectors 30 and 32 of different thermal masses. Theresistance temperature detectors are connected to a bridge circuit (notshown) which maintains the temperature of detector 30 at a fixed levelabove the temperature of detector 32 during the flow measurementprocess. Detector 32 thus acts as a "sensor" of the temperature of thegas flowing past it. The cooling effect of the gas flowing past detector30 causes an electrical power loss through detector 30 and acorresponding drop in current through detector 30. The amount of currentrequired to maintain the temperature of detector 30 at a fixed amountabove the temperature of detector 32 is proportional to the velocity ofthe gas flowing past detector 30. The bridge circuit (not shown) istemperature compensated by selecting resistances for each of theelements of the bridge so that for the same air velocity over a widerange of temperatures the same amount of current is drawn by detector 30to maintain it at the fixed level above the temperature sensed bydetector 32.

Located between detectors 32 and 30 is a nozzle 40. The nozzle 40increases the local velocity of the gas past detector 30. This isadvantageous since the accuracy of detector 30 increases when thevelocity of the gas flow is above some minimum value where the naturalconvection of heat becomes negligible. Also, the nozzle 40 causes theflow downstream to have a flatter, more uniform velocity profile, whichprovides more accurate totalization of flow in passage 16. The diameterof nozzle 24 and the ratio of its diameter to the diameters of thenozzles in valve 24 is selected based upon the maximum and minimum flowrates desired to be measured by the flowmeter for each of its ranges.

The valve 24, which is located in the primary passage 14, includes twoorifices of different diameter, each of which is a diameter less thanthe diameter of primary passage 14. The valve 24 is a cylinder locatedwithin the primary passage 14 with two separate bores spaced from oneanother along the axis of the cylinder and oriented at an angle with oneanother. Valve 24 can be rotated so that either of the orifices 42, 44defined by the bores is aligned with primary passage 14, or so that theentire flow through primary passage 14 is blocked and diverted throughthe secondary passage 16. Valve 24 includes a seal 46, which can be ofany suitable material such as Teflon, for preventing any flow past valve24 and through passage 16 when the valve 24 is in the position forblocking all of the flow through the primary passage 14. Since each ofthe orifices 42, 44 in valve 24 has a diameter less than the diameter ofprimary passage 14, each orifice serves as a restriction to the primaryflow through passage 14 creating a pressure drop between the inlet end20 and the outlet end 22 of secondary passage 16. This insures that afraction of the flow will be diverted through secondary passage 16. Theratio of the gas flow through secondary passage 16 to that throughprimary passage 14 when one of the orifices of valve 24 is in alignmentwith primary passage 14 is generally proportional to the ratio of thecross-sectional area of nozzle 40 to the cross-sectional area of therespective orifice 42 or 44 in use. This flow ratio is highly repeatableover a wide range of flow rates.

In operation the flow to be measured enters flowmeter 10, in thedirection shown by arrow 11, through primary passage 14. If the flow isat a relatively high flow rate, valve 24 is rotated so that orifice 42defined by the bore through valve 24 is aligned with the central boredefining the primary passage 14, as illustrated in FIG. 1. In thismanner, a large portion of the total flow passes through primary passage14 with only a relatively minor amount of the flow passing throughsecondary passage 16 where the flow measuring device is located. Theratio of the flow rates through the two passages is proportional to theratio of the areas of the orifices in the two passages, specificallynozzle 40 and orifice 42. Thus, when a relatively high flow rate isbeing measured, resistance temperature detector 30 measures the flowrate through the secondary passage 16 in the manner described above.From this measurement, the total flow rate can be computed based uponthe area ratio. To insure more precision in the measurement of the totalflow rate, the flowmeter is calibrated over the entire range of flowrates from known calibration standards.

When a relatively low rate of flow must be measured, valve 24 is rotatedso that all of the flow is diverted from the primary passage 14 to thesecondary passage 16. In a like manner, when an intermediate rate offlow must be measured, valve 24 is rotated so that orifice 44 in valve24 is aligned with the primary passage 14. It should thus be apparentthat valve 24 may be utilized in the manner described above to insurethat even though the total flow rates to be measured may vary over awide range, the flow rate through the secondary passage 16 can bemaintained over a relatively narrow range. Thus, regardless of theactual flow rate through the flowmeter 10, the velocity of the gasexiting nozzle 40 does not vary over a wide range. Since the calibrationcurve of current draw of resistance detector 30 versus the velocity ofgas exiting nozzle 40 is a non-linear function, the maintenance of thevelocity of gas exiting nozzle 40 over an optimum range of sensitivityand repeatability improves the accuracy of measurement.

Any one of a number of conventional displays can be used to provide theoutput of resistance detector 30, including the use of conventionalcircuitry to provide a digital display or conventional analog electricalmeters. Also, conventional circuitry can be used to provide a linearoutput signal.

While the embodiment shown in FIG. 1 illustrates a multi-range flowmeterwith a resistance temperature detector as the flow measuring device, itshould be apparent that other types of conventional flow measuringdevices located within secondary passage 16 would function equally aswell. For example, the resistance temperature detectors 32 and 30 couldbe replaced by static pressure sensing devices which would sense apressure drop across the restriction caused by orifice 40.

In another embodiment, a conduit could be used to define the secondarypassage 16 and two resistance coils could be wrapped around the conduit.A third coil would be placed between the two coils and would be used toapply a fixed amount of heat to the flowing gas. When this type of flowmeasuring device is used, a nozzle or other flow restrictor is not usedin the secondary passage. The flow is maintained laminar through thesecondary passage by properly selecting the diameter and length of thesecondary passage. This type of flow measuring device is more fullydescribed in U.S. Pat. No. 2,727,976 to Laub. When this type of flowmeasuring device is utilized it may be desirable to use a laminar flowelement in the primary passage, such as described in U.S. Pat. No.3,792,609 to Blair. A modification of this type of flow measuring deviceuses two heat supplying coils wrapped around the conduit and measuresthe temperature differential of the coils, which is directlyproportional to the mass flow of the fluid. This type of device isdescribed in U.S. Pat. No. 3,938,384 to Blair.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations of those embodiments will occur to those skilled in the art.However, it is to be expressly understood that such modifications andadaptations are within the sphere and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. A dual-range flowmeter for measuring the massflow rate of a gas comprising:a housing having a central boretherethrough defining a primary passage for the gas and at least onesecondary bore defining a secondary passage of lesser diametercommunicating at its ends with the primary passage; a sensor locatedwithin the secondary passage for measuring the rate of mass flow of gasthrough the secondary passage, said sensor including a resistancetemperature detector immersed in the flow in the secondary passage,means for maintaining the resistance temperature detector at a generallyconstant temperature differential above the temperature of the flowinggas and means coupled to the resistance temperature detector for sensingthe electrical power loss of the resistance temperature detector causedby the cooling effect of the flowing gas; a nozzle located within thesecondary passage for increasing the velocity of the gas flowing pastthe resistance temperature detector; and a valve located within thecentral bore between the locations where the ends of the secondarypassage communicate with the primary passage, said valve beingcylindrically shaped, rotatable about an axis generally perpendicular tothe longitudinal axis of the central bore, and having a bore orientedgenerally perpendicular to the longitudinal axis of the cylindricallyshaped valve, whereby when the valve is rotated so that the bore throughthe valve is aligned with the primary passage gas flows through theprimary passage and when the valve is rotated so that the bore throughthe value is not aligned with the primary passage all of the gas flowsthrough the secondary passage.